Select your sub to suit your amp.
Car amps often perform quite badly on the lowest impedance in the specification.

It sometimes helps the sound quality to use a driver with twice the impedance quoted as the minimum. This reduces the power that comes from the amp, but this is often compensated by improved sensitivity of the higher impedance driver. i.e. using an 8ohm driver off a 4ohm amp might be just as loud and WILL sound cleaner.